Example of the present invention relates to a Doherty amplifier, a signal amplification system including the same, and a method for amplifying a signal.
With the compactness and multifunction of mobile communication terminals, many approaches for increasing the operating time of a battery for the terminals have been researched and developed. Of those, there has been a lot of research into an approach for increasing the efficiency of a radio frequency (RF) power amplifier accounting for most of the power consumption of a mobile communication terminal and a representative solution is a Doherty amplifier. However, existing communication protocol standards employ a wide bandwidth while the Doherty amplifier using a λ/4 line (hereinafter, λ refers to a wavelength) uses a narrow bandwidth, and therefore, it is very difficult to achieve high amplification efficiency.
In addition, since the Doherty amplifier uses a carrier amplifier and a peaking amplifier connected in parallel, it is sensitive to a phase difference between the two amplifiers and to the magnitude of a signal of each amplifier. Also, the Doherty amplifier connects the two amplifiers using a quarter wave transformer, thus fundamentally having a narrow frequency band. Moreover, the quarter wave transformer is implemented using a micro strip line, the size of which is very big for high frequency, and therefore, there is a limit to decreasing the size of the quarter wave transformer and the Doherty amplifier including the same.